Multi-phase linear electromagnetic motors generally comprise a row of permanent magnets to generate magnetic flux. Induction coils or phase coils are arranged facing the magnets and in directions that are transverse to the flux lines generated by the magnets. A typical multi-phase linear motor for driving a motion stage may have two or three coils arranged longitudinally, in such a way as to be arranged in line with the motion directions of the motion stage. Hence, the coils are arranged parallel to the motion direction. In a two-phase motor where there are two sets of phase coils, the two phase currents are mutually offset by 90°. In a three-phase motor, the three phase currents are mutually offset by 120°.
A planar motor which comprises linear drivers may drive motion along two chosen motion axes independently such as a main axis or X-axis, and an auxiliary or Y-axis orthogonal to the X-axis. Typically, such a dual-axis planar motor providing independent motion control requires two sets of phase coils, two sets of magnets and two motor drivers. An example of a planar motor which drives motion along both the X-axis and the Y-axis is U.S. Pat. No. 6,949,845 B2 entitled “Planar Motor” which discloses a planar motor providing highly accurate positioning by disposing X-axis and Y-axis coreless type linear motors on the same plane. Two pairs of permanent magnets are disposed orthogonally with respect to two orthogonal motion axes of a movable stage. The permanent magnets generate magnetic flux lines in directions which are perpendicular to a surface of the movable stage. Two pairs of coils are provided such that they oppose and match the two pairs of permanent magnets. This form of planar motor needs two separate sets of coils and magnets, which is therefore not cost-effective in terms of the components required. It also requires two motor drivers in order to provide independent control along the two orthogonal axes. Furthermore, the temperature of the motor fluctuates when the motion profile is changed or the motor stops. When the planar motor runs for a prolonged period, the mechanical components expand and cause thermal drift. This results in inaccurate positioning operation of the planar motor along the two axes of motion.
US Patent Application Publication Number 2008/0246348 A1 entitled “Ironless Magnetic Linear Motors Having Levitating And Transversal Force Capacities” discloses a planar motor which uses one set of three coils and one set of magnets to generate forces along the X-axis, Y-axis and Z-axis to bring about motion along each axis. The forces acting along the respective axes can be controlled independently. However, during the Z-axis motion, an overlap portion between the magnets and the three coils changes along the Z-axis, hence changing the force constant for driving the coils in the X-axis. Thus, although fewer components are required compared to the planar motor described in the abovementioned US patent, the force constant of the main travel axis changes with concurrent movement of the coils along the auxiliary axis. This ultimately affects the positioning control performance of the planar motor. Furthermore, the temperature of the planar motor cannot be kept constant which results in thermal drift and contributes to inaccurate positioning control.
It is therefore desirable to devise a planar motor providing concurrent motion along two orthogonal axes that has improved positioning control.